Mobile apps have become an embarrassment of riches for iPhone and Android. In a world with over 500,000 iPhone apps and over 250,000 Android apps, the toughest part is finding the most useful stuff.

For iPhone users, I'm going to throw you an assist by sharing my top 20 (this is an update of my 2010 iPhone list, and I will update my top Android picks next week). My iPhone picks are all third-party apps that can help you be more productive, streamline regular activities, reduce the number of gadgets in your life, and take advantage of the top benefits that mobile computing has to offer.

Dropbox is a great cloud service that automatically syncs a folder of files between multiple computers (Windows, Mac, or Linux). This app extends Dropbox to the iPhone and includes a built-in reader within the app for PDFs, image files, and Microsoft Office files.

Once you get used to typing on a virtual keyboard (and it honestly took me over a year to do it), then these devices are great for note taking, and Evernote is a great note taking app. It is similar to Dropbox in that it saves data locally but syncs it across all your machines and devices.

There are a ton of to-do apps on iPhone but I prefer Due for its simplicity and its audio alerts. However, this is an iPhone-only task list. If you want something that can sync with your PC, Mac, or the Web, then try 2Door Things.

I love Tripit. It is by far the best app I've found for keeping track of all my travel itineraries. It is powered by some excellent backend systems. You simply forward your confirmation emails (or use the Gmail plugin to do it automatically) for your flights, hotels, rental cars, and more to Tripit and it automatically organizes them into trips with all your details and confirmation numbers.

The official Twitter app (formerly known as Tweetie) is still the best Twitter client on iPhone (although Osfoora is catching up). Twitter itself is an amazing instant-intelligence engine. Two other great social media apps for iPhone are Google+ and Foursquare.

Twitter has largely replaced RSS for me for finding and filtering the latest news. However, I still track some RSS feeds and the best tool I've found to do it with is Reeder. It syncs with Google Reader so it's easy to flip between the mobile app and the desktop, plus the app lets you share to Twitter (and Facebook) and save to Instapaper and ReadItLater.

I've never fully warmed up to the Amazon Kindle e-reader, but I'm a big fan of the Kindle iPhone app. Since it was released I've read a lot more books simply because my phone is always with me and I can pull it out and read a few pages anytime I've got a couple minutes free. Alternatives: Nook, iBooks, and Kobo.

As much as I like the Kindle ebooks, I actually consume more books as audiobooks via Audible. In the past you could download these and sync them via iTunes. But Audible now has its own app, which lets you connect to your Audible library and download over the air, and even gives you a self-contained player optimized for audiobooks.

Sure, you can use iTunes and the built-in iPod app to listen to podcasts, but if you're an avid podcast listener (I regularly follow This Week in Tech, Buzz Out Loud, and Tech News Today) then the app Podcaster offers a better experience. You can download over the air (so that you don't have to constantly sync to a computer to get the latest episodes), you can skip forward and backward 30 seconds, you can increase playback speed to 1.5 times normal speed, and the app is even compatible with AirPlay.

Photoshop is, of course, the best known photo editor in the world and its mobile app doesn't do anything to hurt that reputation. But while the desktop version is known for having a zillion features, the mobile app is distinguished by its simplicity. It's the best iPhone photo editing app for simple crops, brightness adjustments, and sharpens, for example. However, once editing is built into the native Camera app in iOS5, editors such as Photoshop Mobile may quickly become unnecessary.

Pano makes it easy to take excellent panoramas with the iPhone. It helps you line up your shots and it automatically corrects many of the imperfections. My wife is a photographer with a big, expensive Nikon camera and she's regularly jealous of some of the shots I can get with the iPhone and Pano.

The camera and camera app on the iPhone are now good enough to replace a point-and-shoot. You can even take pictures that are worthy of saving in your family albums. For those, I upload them to Flickr using the iPhone app. For the everyday photos that I just want to quickly post on social media, I use Instagram. It is very quick, dead simple to use, and very social media friendly. But, do me a favor and go easy on the filters. They are badly overused by most Instagram users, while 90% of photos are better with no filter at all.

I used to carry a separate Garmin GPS unit for turn-by-turn directions but I eventually got rid of it and decided to just use the iPhone instead. In researching the various apps, I eventually decided on NAVIGON, which is a company that makes a lot of the built-in navigation systems for many cars. Tip: Make sure your iPhone is plugged in to power when you run a GPS navigation program like this because otherwise it will quickly drain your battery.

A great companion to a GPS system is the app "Where To?" which lets you quickly look up various types of shops and services, from Cuban restaurants to medical specialists to animal hospitals to local museums and much more.

This is a great little app that can save you from buying a pedometer. It uses the iPhone's GPS to track the miles you've run or walked, and it compiles the data into some nice dashboards that you can view on your phone or on RunKeeper's website.

Another app for all of you health-conscious geeks out there is Nutrition Menu. This thing is a mobile compendium of nutritional data. It has calorie information on common foods and most major restaurants, and it has calorie-burning information for many types of exercises. It also allows you to track your daily weigh-ins and makes notes on your progress.

This is an app plus a website and you can quickly sync between the two. The way it usually works for me is that my wife makes a grocery list, enters it into grocerygadgets.com, and then it syncs to my iPhone so that I can swing by the grocery store and pick up the stuff.

This is a great app for shoppers. It turns the iPhone camera into a barcode scanner and it's quite accurate. You simply scan a product's UPC code and let the app go to work to find it in Google Product Search and TheFind. For food it will even look up allergen information and for books it will scan to see if you can get it in a local library. You'll be amazed at how fast it works. A similar product is SnapTell, which not only scans barcordes but you can also take a picture of the cover of a book or DVD and it can look them up that way. These apps are great when you're shopping at a retail store and want to check the prices of products online before buying. It also reads QR codes.

Your picks?

What are your picks for the most useful, valuable, and productive iPhone apps? Post them in the discussion below.

Jason Hiner is Editor in Chief of TechRepublic and Long Form Editor of ZDNet. He writes about the people, products, and ideas changing how we live and work in the 21st century. He's co-author of the upcoming book, Follow the Geeks (bit.ly/ftgeeks).